1. Technical Field of the Invention
This invention related generally to frequency synthesis and more particularly to voltage controlled oscillators.
2. Description of Related Art
Communication systems are known to support wireless and wire lined communications between wireless and/or wire lined communication devices. Such communication systems range from national and/or international cellular telephone systems to the Internet to point-to-point in-home wireless networks. Each type of communication system is constructed, and hence operates, in accordance with one or more communication standards. For instance, wireless communication systems may operate in accordance with one or more standards including, but not limited to, IEEE 802.11, Bluetooth, advanced mobile phone services (AMPS), digital AMPS, global system for mobile communications (GSM), code division multiple access (CDMA), local multi-point distribution systems (LMDS), multi-channel-multi-point distribution systems (MMDS), and/or variations thereof.
Depending on the type of wireless communication system, a wireless communication device, such as a cellular telephone, two-way radio, personal digital assistant (PDA), personal computer (PC), laptop computer, home entertainment equipment, et cetera communicates directly or indirectly with other wireless communication devices. For direct communications (also known as point-to-point communications), the participating wireless communication devices tune their receivers and transmitters to the same channel or channels (e.g., one of the plurality of radio frequency (RF) carriers of the wireless communication system) and communicate over that channel(s). For indirect wireless communications, each wireless communication device communicates directly with an associated base station (e.g., for cellular services) and/or an associated access point (e.g., for an in-home or in-building wireless network) via an assigned channel. To complete a communication connection between the wireless communication devices, the associated base stations and/or associated access points communicate with each other directly, via a system controller, via the public switch telephone network, via the Internet, and/or via some other wide area network.
For each wireless communication device to participate in wireless communications, it includes a built-in radio transceiver (i.e., receiver and transmitter) or is coupled to an associated radio transceiver (e.g., a station for in-home and/or in-building wireless communication networks, RF modem, etc.). As is known, the transmitter includes a data modulation stage, one or more intermediate frequency stages, and a power amplifier. The data modulation stage converts raw data into baseband signals in accordance with a particular wireless communication standard. The one or more intermediate frequency stages mix the baseband signals with one or more local oscillations to produce RF signals. The power amplifier amplifies the RF signals prior to transmission via an antenna.
As is also known, the receiver is coupled to the antenna and includes a low noise amplifier, one or more intermediate frequency stages, a filtering stage, and a data recovery stage. The low noise amplifier receives inbound RF signals via the antenna and amplifies then. The one or more intermediate frequency stages mix the amplified RF signals with one or more local oscillations to convert the amplified RF signal into baseband signals or intermediate frequency (IF) signals. The filtering stage filters the baseband signals or the IF signals to attenuate unwanted out of band signals to produce filtered signals. The data recovery stage recovers raw data from the filtered signals in accordance with the particular wireless communication standard.
As is further known, the one or more intermediate frequency stages of the receiver and of the transmitter each include a local oscillator to produce a local oscillation that is mixed with an incoming signal to produce an output of the intermediate frequency stage. Typically, a local oscillator includes a phase locked loop (PLL) to produce an oscillation that is used as the local oscillation or is processed to produce the local oscillation. A popular PLL architecture includes a phase and/or frequency detector, a charge pump, a loop filter, a voltage controlled oscillator (VCO), and a feedback module.
In operation, the phase and/or frequency detector compares phase and/or frequency of a reference oscillation (which is typically produced by a crystal oscillator) and a feedback oscillation (which is produced by the feedback module by dividing the output oscillation by a divider value). If the phase and/or frequency detector detects a difference between the phase and/or frequency of the reference oscillation and the feedback oscillation, it generates a difference signal. The difference signal may be a positive signal when the phase and/or frequency of the feedback oscillation leads the phase and/or frequency of the reference oscillation, which occurs when the output oscillation is greater than the desired rate. The difference signal may be a negative signal when the phase and/or frequency of the reference oscillation leads the phase and/or frequency of the feedback oscillation, which occurs when the output oscillation is less than the desired rate.
The charge pump is operably coupled to convert the difference signal into a positive current signal or a negative current signal depending on the state of the difference signal. The loop filter converts the positive or negative current into a control voltage, which is provided as the input to the VCO. The VCO generates the output oscillation based on the control voltage. For instance, the rate of the output oscillation increases as the control voltage increases due to a positive current, the rate of the output oscillation decreases as the control voltage decreases due a negative current, and the rate of the output oscillation remains constant for a constant control voltage.
There are a variety of ways to implement a voltage control oscillator including a ring oscillator and an inductor-capacitor based oscillator. In many radio transceiver applications, the inductor-capacitor based oscillator is used due to the high frequencies that need to be generated (e.g., 900 MHz to 5.75 GHz). When implemented on an integrated circuit, the inductor-capacitor based VCO typically provides a 10% frequency tuning range (i.e., the output oscillation range from minimum control voltage to maximum control voltage) once a local oscillation for a particular channel frequency has been selected (e.g., 2.4 GHz, 2.42 GHz, 2.44 GHz, etc.). To change the local oscillation for different channel frequencies, the capacitance of the inductor-capacitor based VCO is changed. For some radio transceiver applications, the tuning range needs to be about 0.5%.
Therefore, a need exists for a precision tunable voltage controlled oscillator (VCO) that has an accurate and small tuning range.